A thin film transistor liquid crystal display (TFT-LCD) device has characteristics of fast response, high contrast, high color fidelity and so forth, and occupies the majority of the LCD market.
As shown in FIG. 1, a display panel of the TFT-LCD includes a plurality of gate lines 11 and a plurality of data lines 12, and a plurality of display units defined by the gate lines 11 and the data lines 12. Each of the display units is provided with a thin film transistor (TFT) 20 and a pixel electrode 13. A display principle of the TFT-LCD is as follows: the gate lines 11 and the data lines 12 control ON/OFF states of each TFT 20 and an image signal is written into a corresponding pixel electrode 13 so as to achieve the display. In the practical application, in order to ensure the image signal to be written into the pixel electrode of the display unit instantly and accurately, an on-state current of the TFT needs to be large; and in order to increase brightness of the displayed image, an aperture ratio of the display unit needs to be large. Both the on-state current and the aperture ratio of the display unit are affected by a width to length ratio of a channel of the TFT.
A general structure of the TFT 20 is shown in FIG. 2, which includes a gate electrode 21, a source electrode 22 and a drain electrode 23. A width W of the channel of the TFT is a total length of surfaces where the source electrode 22 and the drain electrode 23 face to each other. A length L of the channel of the TFT is a distance between the source electrode 22 and the drain electrode 23. Another structure of the TFT 20 is shown in FIG. 3, wherein the source electrode 22 is of a U shape, the drain electrode 23 is of a rectangular shape. In such structure, the length L of the channel of the TFT remains the same, and the width of the channel of the TFT is W=W1+W2+W3.
Specifically, a calculation formula of the on-state current is as follows: Ion=½ μCW/L(Vg−Vt)2, where Ion is the on-state current, μ is a carrier mobility, C is a capacitance of the gate electrode of the TFT, W is the width of the channel of the TFT, L is the length of the channel of the TFT, Vg is a positive voltage of the gate electrode of the TFT, and Vt is a threshold voltage of the TFT. As can be seen from the formula, the on-state current will be increased along with the increase of the width and length ratio W/L of the channel of the TFT in the case that other parameters remain unchanged. In addition, since it is difficult to make the length L of the channel of the TFT to be very small due to the limitation of lithography accuracy, increasing the width W of the channel of the TFT has become an important method to increase the on-state current. However, the width W of the channel of the TFT cannot be increased unlimitedly, because along with the increase of the width W, an area of the TFT is increased, which will cause the aperture ratio of the display unit to be decreased and then the display brightness to be decreased, wherein the aperture ratio of the display unit is equal to a ratio of an area of a light transmission region to that of the display unit, the light transmission region is a region of the display unit other than a non-transparent region such as the TFTs.